Usher syndrome, or Usher's syndrome, is an inherited condition that is a leading cause of deaf-blindness. People born with this syndrome gradually become blind and deaf, usually by the age of thirty. In more severe cases, children and even infants may have significant impairment of their vision and hearing, as well as difficulties maintaining their balance, due to problems in the vestibular system. Usher syndrome is an autosomal recessive disorder of combined deafness and blindness resulting in one of the most debilitating forms of retinal degeneration, since it affects patients who already suffer from deafness. Usher type 1B is due to mutations in the MYO7A gene that encodes an unconventional myosin expressed in the RPE (retinal pigment epithelium) and photoreceptor cells, within the retina, plus other cells of the body, including the cochlear hair cells. Myo7a-null mice have mutant retinal phenotypes, including defects in phagosome and melanosome transport.
Mutations in the MYO7A gene account for approximately 60% of cases with a clinical diagnosis of Usher Syndrome Type I. Mutations in the USH2A gene accounts for approximately 80% of cases with a clinical diagnosis of Usher Syndrome Type II.
MYO7A has been reported to be double headed myosin. It consists of a conserved myosin motor domain, a neck region with 5 IQ motifs, a short coiled-coil domain, and a tail consisting of two repeats of a myosin tail homology domain (Myth4) and a band 4.1 ezrin/radaxin/moesin homology domain (FERM) separated by a poorly conserved S113 domain.